1. Field of Invention
The invention relates to a diode, and, in particular, to a light emitting diode.
2. Related Art
Light emitting diodes have been merchandised since 1960. Because the light emitting diode can withstand the great vibration, has the long lifetime and consumes less power, it is now widely applied to various products, such as indicators or light sources of home appliance products and various instruments. Recently, the light emitting diode is further applied to the outdoor display, such as a large-scale outdoor display and a traffic signal lamp, in response to the developments of colorfulness and luminance.
In addition, manufacturers further develop high luminance blue, green or red light emitting diodes, or white light emitting diodes in response to the requirements of the user in full-color and high luminance in order to provide better color display effect and higher luminance requirement.
However, the light emitting diodes developed according to various applications still have many problems to be solved. The problems may be, for example, the poor lighting efficiency and the uneven wavelengths of the light emitting diodes.
As mentioned hereinabove, the problem of the poor lighting efficiency of the light emitting diode occurs because the conventional light emitting diode is restricted by the lateral wave guiding phenomenon and the total internal reflection on the crystal surface of the light emitting diode. Thus, the light outputted from the light emitting diode cannot be effectively transferred out of the crystal. That is, a great portion of the output light of the conventional light emitting diode is wasted.
In order to solve the problem of the poor lighting efficiency caused by the total internal reflection, the manufacturers have roughened the crystal surface so as to enhance the lighting efficiency of the light emitting diode. One of the frequently used methods of roughening the surface is to randomly roughen the surface by way of dry etching. Alternatively, an electron-beam lithography method or a light interfering method can be used to form a photon crystal so as to make the crystal surface have a patterned surface roughness.
Referring to FIG. 1, a conventional light emitting diode I mainly includes a substrate 10, an N-type epitaxial layer 11, a light emitting layer 12, a P-type epitaxial layer 13 and a transparent conductive layer 14. A first electrode 111 for the electrical connection is disposed on the N-type epitaxial layer 11, and a second electrode 141 for the electrical connection is disposed on the transparent conductive layer 14. The P-type epitaxial layer 13 is disposed on the light emitting layer 12 and has one surface 130 roughened by an etching process. The transparent conductive layer 14 is disposed on the P-type epitaxial layer 13 and has one surface 140 roughened by another etching process. Thus, the total internal reflection phenomenon can be decreased so that the lighting efficiency can be enhanced according to the surface roughening processes for the P-type epitaxial layer 13 and the transparent conductive layer 14.
After the surface is roughened using the conventional etching process, the lighting efficiency of the light emitting diode can be increased. However, the lighting efficiency cannot be greatly increased. If the crystal surface is roughened by forming the photon crystal, the lighting efficiency of the light emitting diode can be increased more effectively, but the required machine apparatus is more expensive and the mass production cannot be made. That is, this method has higher cost.
Consequently, it is an important subject of the invention to provide a light emitting diode having extremely high lighting efficiency and low cost.